Radio beams are generally associated with tools for providing assistance in operation and maintenance. In general, the diagnosis performed by these systems on the radio beam comprises no more than handling a set of alarms for detecting non-operation of various elementary functions of the beam.
These prior systems are inaccurate when it is desired to obtain information for a given leg in a given channel concerning, inter alia:
propagation events (essentially multiple paths), together with statistics on these events as a function of their characteristics;
the channel transfer function and the consequent residual linear distortion, together with its possible slow change over time;
residual non-linear distortion; and
the signal-to-noise ratio on the received signal.
Propagation events can be analyzed in a manner which is known per se by recordings making use of equipment dedicated to this task (radio link associated with a radio beam analyser). This has the drawback that while the analysis is taking place, the link equipped in this way is unusable for normal transmission purposes.
The analysis can be performed non-intrusively, i.e. without disturbing the operation of the link, by analyzing the coefficients of a delay equalizer if the beam is fitted with one. In this case, the drawback is that the analysis is limited by the characteristics of the delay equalizer. If the analyser is synchronous, then its analysis frequency window does not cover the entire signal spectrum. In addition, the frequency resolution of the analysis is limited in this case by the number of coefficients associated with the equalizer.
This method is also applicable to any other countermeasure circuit with which a digital radio beam may be fitted (frequency equalizers, diversity receivers), and it always suffers from analyzing the channel only approximately. In addition, the analysis performed always relates to distortion as corrected and never to residual distortion.
The invention seeks to remedy these drawbacks by providing a non-intrusive system for diagnosing a transmission channel for a digital modem, the system being applied to a demodulator regenerator, and being capable, inter alia, of analyzing:
the transfer function of a radio beam channel at high frequency resolution, thereby enabling multiple paths to be finely analyzed as well as analyzing modem filtering and the way it changes over time;
the residual non-linear distortion on reception;
the signal-to-noise ratio in the received signal;
the root mean square error in the received signal; and
quadrature defects in the constellation.